1. Field of the Invention
The present invention relates to the generation of waveforms for use in radar applications, and more particularly, to the generation of non-linear FM waveforms for use in digitally implemented low frequency radar.
2. Discussion of the Prior Art
Radar is an electromagnetic system for the detection and location of objects or targets. The basic concept underlying radar is the capture and analysis of a particular waveform which is transmitted from a source towards a target and reflected back towards the source. This reflected signal or echo is analyzed to determine certain information including the distance to the target, the position of the target, and the targets velocity. The distance to the target is calculated as a function of elapsed time between the transmission of the particular waveform and the reception of the reflected signal. The direction or angular position of the target is determined from the direction of arrival of the reflected waveform. If either the target or radar, or both are moving, i.e. relative motion, the shift in the carrier frequency of the reflected waveform is a measure of the target's relative velocity. This frequency shift is known as the Doppler shift principle and is well known in the art.
Radar can take on many forms and be utilized in a multitude of applications in the civil and military arena. Civil radar applications include air traffic control, aircraft and ship navigation, remote sensing applications including geological activity monitoring and weather front monitoring, and law enforcement i.e. the detection of speeding motorists. Many of the military radar applications are similar to the civil applications; however, the more traditional role of radar in military applications is in surveillance, navigation, and the control and guidance of weapons.
In the initial stages of radar development, radar was implemented utilizing analog components. However, with the advent of reliable digital processing techniques, radar is now implemented utilizing digital computers or digital processors. Digitally implemented radar does not per se provide superior results over their analog counterparts; however, digitally implemented radar are typically more reliable, require less continuous adjustment, and provide information more efficiently and in a more convenient format.
Current radars employ the use of a weighted function, such as Hamming or Dolph-Chebychev, in digital pulse compression to lower the range-time sidelobes to a desired level thereby minimizing false target detection. For example, a linear FM signal which is unweighted will result in a compressed pulse with a mainlobe to peak sidelobe level of only 13 dB. Applying Hamming weights to the reference linear FM signal will result in a compressed pulse having an improved mainlobe to sidelobe ratio of 43 dB. These results apply to a 204.8 microsecond pulse sampled at 5 MHz and having a 4 MHz signal bandwidth. However, having this weighting function applied to the reference linear FM signal creates a loss in the reference signal as well as a mismatch between the reference and receive spectrums. This causes a 1.34 dB loss in signal to noise while also broadening the compressed pulsewidth. A 1.34 dB loss in signal to noise translates to a decrease in detection range of approximately eight percent which is a significant decrease in any radar application.